Rotel RB 993 humming

MichaelB · 8th January 2006, 01:57 PM

Hi,

I've recently purchased a used Rotel RB-993 3 channel amp on eBay
to accompany my RB-1080 for surround speakers.

Performs well but there is always a gentle humming noise (yes,
humming, no hiss...) coming from all three speakers. Not very
loud but clearly audible from all surround speakers in silent dvd
scenes.

I've checked cabling, transformer, everything, only speakers attached,
no change. Humming starts immediatly when the output relays turn on.
Power-supply seems to be ok, 74V DC with roughly 35mV AC on top
with no load.

Since the amp is a class-2 device (no earth at all) bad earth can't be
the reason. The transformer itself is dead silent, too.

Any ideas what could be the cause for the humming?
Maybe somebody experienced such effects with this amp before?

A pointer to the service manual might be helpfull, too.

pinkmouse · 8th January 2006, 02:05 PM

Sounds like noise/ripple coming through from the PSU. You may have a dried up cap or faulty bridge diode. It may be a faulty component on the amp PCB, but as it is affecting all channels, that is less likely.

inteificio · 9th January 2006, 09:43 PM

Hi,

are you sure this is not grounded?

I own one of theese wonderful amps as well. I got a buzzing aswell whenever my PC was plugged in (whether on or off).

I cut the ground wire in the 993, no buzz.

I am considering fitting a MOV in the gap but that should return it to being safe.

Check again that your unit has no ground, I am pretty sure mine did. If not how the hell did breaking the ground do anything if it was not there to start with!

anatech · 9th January 2006, 10:06 PM

Hi MichaelB,
Does it hum if the inputs are not plugged in? Do you have cable hooked up?

-Chris

mastertech · 9th January 2006, 10:12 PM

Anatech by the sound of it is not a ground loop, of course they are hooked up
whats your diagnosis

cheers

anatech · 10th January 2006, 01:00 AM

Hi mastertech,
One never can tell until the basic tests are done. If cable is involved, the cable ground can be the largest, most evil ground loop ever designed.

So it's time to gather information.

-Chris

mastertech · 10th January 2006, 03:20 AM

what if isnt that (grd loop) and there is still some light hum as the
fellow has said on very low audio levels, whats your diagnosis

MichaelB · 10th January 2006, 06:40 AM

Hi,

the mains plug has no earth contacts, so it can't be an earth loop.

No input cables attached, only speakers, there is humming from the
speakers. Having my pre connected or not makes no difference.

The source for the hum has to be somewhere in the amps circuitry,
most likely the PSU, as pinkmouse suggested, since all three channels
produce humming noise.

I will check the PSU more closely as soon a I have some spare time
again.

Does anybody by chance have the service manual available?

Tube_Dude · 10th January 2006, 06:43 AM

Is the signal ground connected to the case?

inteificio · 10th January 2006, 07:14 AM

just one extra. After a quick chat fitting an MOV in line with earth does not make it safe, well safer than not at all.

8th January 2006, 01:57 PM	#1
MichaelB diyAudio Member Join Date: Apr 2003 Location: Paderborn, NRW	Rotel RB 993 humming Hi, I've recently purchased a used Rotel RB-993 3 channel amp on eBay to accompany my RB-1080 for surround speakers. Performs well but there is always a gentle humming noise (yes, humming, no hiss...) coming from all three speakers. Not very loud but clearly audible from all surround speakers in silent dvd scenes. I've checked cabling, transformer, everything, only speakers attached, no change. Humming starts immediatly when the output relays turn on. Power-supply seems to be ok, 74V DC with roughly 35mV AC on top with no load. Since the amp is a class-2 device (no earth at all) bad earth can't be the reason. The transformer itself is dead silent, too. Any ideas what could be the cause for the humming? Maybe somebody experienced such effects with this amp before? A pointer to the service manual might be helpfull, too.

8th January 2006, 02:05 PM	#2
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	Sounds like noise/ripple coming through from the PSU. You may have a dried up cap or faulty bridge diode. It may be a faulty component on the amp PCB, but as it is affecting all channels, that is less likely. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

10th January 2006, 06:43 AM	#9
Tube_Dude diyAudio Member Join Date: Mar 2002 Location: Aveiro-Portugal	Is the signal ground connected to the case? __________________ Jorge

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9th January 2006, 09:43 PM	#3
inteificio diyAudio Member Join Date: Dec 2004 Location: York	Hi, are you sure this is not grounded? I own one of theese wonderful amps as well. I got a buzzing aswell whenever my PC was plugged in (whether on or off). I cut the ground wire in the 993, no buzz. I am considering fitting a MOV in the gap but that should return it to being safe. Check again that your unit has no ground, I am pretty sure mine did. If not how the hell did breaking the ground do anything if it was not there to start with!

9th January 2006, 10:06 PM	#4
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi MichaelB, Does it hum if the inputs are not plugged in? Do you have cable hooked up? -Chris

9th January 2006, 10:12 PM	#5
mastertech Banned Join Date: Mar 2005 Location: Australia	Anatech by the sound of it is not a ground loop, of course they are hooked up whats your diagnosis cheers

10th January 2006, 01:00 AM	#6
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi mastertech, One never can tell until the basic tests are done. If cable is involved, the cable ground can be the largest, most evil ground loop ever designed. So it's time to gather information. -Chris

10th January 2006, 03:20 AM	#7
mastertech Banned Join Date: Mar 2005 Location: Australia	what if isnt that (grd loop) and there is still some light hum as the fellow has said on very low audio levels, whats your diagnosis

10th January 2006, 06:40 AM	#8
MichaelB diyAudio Member Join Date: Apr 2003 Location: Paderborn, NRW	Hi, the mains plug has no earth contacts, so it can't be an earth loop. No input cables attached, only speakers, there is humming from the speakers. Having my pre connected or not makes no difference. The source for the hum has to be somewhere in the amps circuitry, most likely the PSU, as pinkmouse suggested, since all three channels produce humming noise. I will check the PSU more closely as soon a I have some spare time again. Does anybody by chance have the service manual available?

10th January 2006, 07:14 AM	#10
inteificio diyAudio Member Join Date: Dec 2004 Location: York	just one extra. After a quick chat fitting an MOV in line with earth does not make it safe, well safer than not at all.

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